1. Technical Field of the Invention
The present invention relates to a bookbinding system for producing a booklet wherein an image forming apparatus is connected with a bookbinding apparatus, and all the production operations from printing to bookbinding are performed in one process. 
2. Description of Related Art
There has been widespread use of a bookbinding system wherein a high-speed image forming apparatus such as an electrophotographic image forming apparatus is connected with a bookbinding apparatus, and all the production operations from image formation to bookbinding are performed in one process.
As compared to the conventional general bookbinding process wherein the printing process is separated from the bookbinding process, such a bookbinding system is characterized in that the contents of printing can be changed as desired, although the printing speed and bookbinding speed are lower. This bookbinding system is evaluated as a high-efficiency bookbinding system in a comprehensive manner, and has been used as a POD (Print On Demand) system.
In the POD system, the image forming apparatus is connected with the bookbinding apparatus. Further, the POD system is often placed in an office or the same building as the office, not in a factory specifically designed for printing and bookbinding. This requires a compact configuration of the system as well as the components constituting the system. 
Thus, a preferably used bookbinding apparatus produces a booklet by binding sheets using a comparatively simple process of gluing.
A hot melt adhesive is used as an adhesive. In response to consumption in coating, replenishing adhesive pellets little by little ensures continuous bookbinding operation. The Patent Documents 1 and 2 disclose a replenishing apparatus wherein the coating roller is moved along the spine of the stack of sheets to apply adhesive to the stack of sheets, and a small and constant amount of adhesive pellet (several pieces at a time) is supplied into a molten adhesive tank wherein a coating roller is immersed.
[Patent Document 1] Unexamined Japanese Patent Application Publication No. 2004-209746
[Patent Document 2] Unexamined Japanese Patent Application Publication No. 2004-276457
In a coating mechanism wherein a coating section made up of a coating roller and coating solution tank is moved along the spine of a stack of sheets and adhesive coating is provided, the tank capacity will be limited due to the limited capacity of the coating section. This will result in a sharp reduction of the liquid level in the tank due to the reduction in the liquid volume consumed in coating. To  ensure uniform coating, means must be provided to reduce the shift of the liquid level in the vertical direction. In Patent Document 1, when there is a reduction in the amount of adhesive, a predetermined number of pellets are supplied, whereby the liquid level of adhesive is kept at a constant level. However, according to the method of supplying adhesive in the Patent Document 1, there is a shift in the adhesive liquid level in the vertical direction, which makes it difficult to ensure uniform coating. This is due to the following reasons:
FIG. 1 will be used for this explanation:
The liquid level LS of the molten adhesive is detected using an adhesive volume sensor TS for detecting the temperature. As shown in FIG. 1 (a) and FIG. 1 (b) or FIG. 1 (c), the adhesive volume sensor TS is detached from the liquid level LS by the reduction in the liquid level LS. In this case, the temperature detected by the adhesive volume sensor TS is reduced, and reduction in the liquid level LS is detected.
The following procedure is used to detect the liquid level LS: When the adhesive tank BT has a small capacity, the percentage of the amount of adhesive to be sucked up by the coating roller RL for is large as compared to the amount of  adhesive in the adhesive tank BT. This causes a greater fluctuation of the liquid level LS. If there is a difference in the thickness among the stacks of sheets to be coated, a change in the status from FIG. 1 (a) to FIG. 1 (b) occurs in one coating process, or a change occurs in the status of FIG. 1 (c).
As illustrated, when the coating roller turns in the arrow direction, adhesive liquid level LS is higher on the downstream side in the rotating direction of the coating roller RL due to the viscosity of liquid. However, the liquid level is stable on the downstream side, and detection on the downstream side provides accurate information on the amount of adhesive in the adhesive tank BT.
The adhesive volume sensor TS merely detects a change in the status from FIG. 1 (a) to FIG. 1 (b), and a change in the status from FIG. 1 (a) to FIG. 1 (c). It is not sufficient to distinguish between the status of FIG. 1 (b) and that of FIG. 1 (c). To be more specific, the adhesive volume sensor TS is responsible for on/off detection alone, and is incapable of detecting a reduction in the liquid level. Thus, when adhesive is replenished using the detection signal by the sensor TS of FIG. 1, a shift of the adhesive liquid level occurs in the vertical direction due to  the difference in the rate of consumption of adhesive. This causes difficulties in uniform coating. If a plurality of sensors are used, multi-phased detection of the liquid level may be possible, but a problem will be raised in costs and sensor installation space. More serious problems will occur when the detection accuracy is to be enhanced.